1. Field of the Invention
The present invention relates to a method for estimating the press formability of galvannealed steel sheet by X-ray diffraction and, more particularly, to a method for making an estimation of whether or not galvannealed steel sheets produced by galvanizing and post-heat treatments can provide the excellent press formability, e.g., rustproof steel sheets for car bodies by X-ray diffraction in a non-destructive and continuous manner without being affected by the coating weight.
2. Prior Art
Galvannealed steel sheets having paintablility adhesion of coating, weldability and press formability in addition to the corrosion resistance of galvanized steel sheets have heretofore been produced and used in various industrial fields. Such galvannealed steel sheets are produced by subjecting steel sheets to hot dip galvanizing electro-galvanizing or vapor zinc depositing and then to post-heat treatments, thereby alloying the zinc coating and the steel matrix.
When steel sheets are heat-treated after zinc coating, .gamma., .delta..sub.1 and .GAMMA. phases represented by FeZn.sub.13, FeZn.sub.7 and Fe.sub.5 Zn.sub.21, respectively, are successively formed by the interdiffusion of Fe and Zn with the progress of alloying. In the production of galvannealed steel sheets, the degree of alloying has so far been controlled with the average content of Fe in the coating. At the same time, the average content of Fe in the coating has also been used as an index to control the quality of the galvannealed steel sheets. This is premised on the assumption that the structures (distribution of each phase) of the coating having a close correlation with the quality correspond to the average content of Fe.
According to the inventors' studies so far made, it has been found that because the average content of Fe in the coating does not necessarily coincide with the structures of the coating, it is not possible to have the reasonable quality constantly by using the average content of Fe in the coating as an index. This will now be explained for steel sheets produced by vapor zinc depositing in vacuo and hot-dip galvanizing. After alloying, for instance, both steel sheets contain an average Fe content of 10%, but steel sheets obtained by hot-dip galvanizing includes, in addition to the major .delta..sub.1 phase with the .GAMMA. phase, a limited proportion of the .gamma. phase. On the other hand, the vapor zinc coated steel sheets include, in addition to the major .delta..sub.1 phase without .GAMMA. phase, a thick .gamma. phase. In another instance wherein galvannealed steel sheets are produced from titanium added and aluminium killed steel sheets by hot-dip galvanizing, there is indeed a difference in the thickness of the .gamma. or .GAMMA. phase even at the same average content of Fe.
It is believed that these phenomena occur due to the fact that the alloying of Fe-Zn by heating proceeds on the basis of diffusion under non-equilibrium conditions. Thus, the structures of coating such as the thickness of the .gamma. or .GAMMA. phase differ even at the same average content of Fe. In order to perform quality control using the average content of Fe in the coating as an index, it is required to have the types of base steel and the coating conditions under strictly identical control. Also, considerable difficulty is encountered in controlling the quality of a wide variety of galvannealed steel sheets. Problems with measuring the average content of Fe in coating by chemical analysis are that cutting of galvannealed steel sheets is required for sampling and so much time is required for analysis, and are that feeding back to the alloying treatment is delayed.
In order to meet a recent demand toward making car bodies greatly rustproof, galvannealed steel sheets have increasingly been used. In particular, materials for automotive rustproof steel sheets are now increasingly produced by press forming with severe drawing. From the results of studies so far made of the quality of galvannealed steel sheets and the structure of the coating, it has been found that the structures of the coating, in which a large amount of the .gamma. phase is present on the surface, offers a problem in connection with severe drawing, because the .gamma. phase, formed on the uppermost layer of galvannealed steel sheets, is relatively soft. In other words, it is preferred that the .gamma. phase is reduced as much as possible. This is because, in the case of galvannealed steel sheets with the coating structures in which a large amount of the .gamma. phase is present on the surface, it is so increased in the surface friction with a mold (or die) during drawing that its feeding into the mold gets worse, possibly resulting in the wall break of steel sheet or the sticking of the coating to the mold.
When a large amount of the .GAMMA. phase grows with the progress of alloying while the .gamma. phase disappears from the surface of the coating, on the other hand, there occurs so-called a powdering phenomenon in which, because of the .GAMMA. phase being hard but brittle, the coating layer peels off during press forming. When this powdering becomes vigorous, almost all of the coating layer falls away from the steel sheets, so that the corrosion resistance of the coating does not only deteriorate, but also affects adversely press formability.
Thus, the quantities of the .gamma. and .GAMMA. phases in the coating have a close correlation with the quality, especially, press formability, of galvannealed steel sheets. In order to obtain galvannealed steel sheets with good press formability, the alloying conditions have to be regulated to ensure that the quantities of the .gamma. and .GAMMA. phases grown does not exceed a proper value, estimating the drawability and anti-powdering property after alloying depending upon the purpose of use. Because the amount of the .gamma. phase remaining on the surface of the coating and the amount of the .GAMMA. phase formed in the coating are unknown, aforesaid, usage of the average content of Fe in the coating as an index of degree of alloying is not possible to regulate the alloying conditions to improve press formability.
An object of this invention is therefore to solve the problems incidental to the index of degree of alloying so far used with the aforesaid technique. In order to make it possible to produce galvannealed steel sheets which are used not only for general purposes but also for severe press forming, a proper reference value for press formability is predetermined and, this is immediately fed back to a coating production line. In other words, the present invention has for its object to estimate the press formability of galvannealed steel sheets by using the aforesaid value as an index in a nondestructive and continuous manner without being affected by the coating weight, while the coating production line is running.
When a galvanized steel sheet is subjected to an alloying treatments, the .gamma., .delta..sub.1 and .GAMMA. phases grow successively after the .eta. phase disappears from the surface of the coating. The proportion of the .gamma. or .GAMMA. phase formed vary with other phases depending upon the degree of alloying. The inventors have examined the intensities of X-ray diffraction of the .gamma. and .GAMMA. phases of each of various coating wherein the proportion of each phase differs depending upon the degree of alloying. In consequence, it has been found that the thickness or quantity of the .gamma. and .GAMMA. phases corresponds to the total X-ray diffranction intensities I(.gamma.) and I(.GAMMA.) of the .gamma. and .GAMMA. phases. Further studies have revealed that: ##EQU2## wherein I(.gamma.)=the total X-ray diffraction intensity of the .gamma. phase,
I(.GAMMA.)=the total X-ray diffraction intensity of the .GAMMA. phase, PA1 I.sub.B (.gamma.)=the background X-ray diffraction intensity of the .gamma. phase, PA1 I(.gamma.)-I.sub.B (.gamma.)=the true X-ray diffraction intensity, of the .gamma. phase, and PA1 I(.GAMMA.)-I.sub.B (.GAMMA.)=the true X-ray diffraction intensity of the .GAMMA. phase, PA1 I(.GAMMA.)=the total X-ray diffraction intensity of the .GAMMA. phase, PA1 I.sub.B (.gamma.)=the background X-ray diffraction intensity of the .gamma. phase, PA1 I(.gamma.)-I.sub.B (.gamma.)=the true X-ray diffraction intensity of the .gamma. phase, PA1 I(.GAMMA.)-I.sub.B (.GAMMA.)=the true X-ray diffraction intensity of the .GAMMA. phase, PA1 .GAMMA.=the .GAMMA. phase of an Fe-Zn intermetallic compound in coating of galvannealed steel sheets, and PA1 .gamma.=the .gamma. phase of the intermetallic compound, are used as a index to determine press formability; the former being used to estimate drawability and the latter to estimate anti-powdering property.
have a good correlation with the quality of a product; the former with the (outer diameter) ratio of the diameter of the disc before drawing to the diameter of the flange after drawing when the draw depth is kept constant in cup drawing test and the latter with the powdering amount in peeling test after bending and bending-back. The above-defined proportions, calculated from the X-ray diffraction intensities of the .gamma. and .GAMMA. phases measurable on or apart from the coating production line, can be used as an index to estimate press formability, thereby attaining the aforesaid object.
More specifically, the present invention provides a method for estimating the press formability of galvannealed steel sheets, wherein: ##EQU3## wherein I(.GAMMA.)=the total X-ray diffraction intensity of the .gamma. phase,